1. Field of the Invention
The present invention concerns a method for generation of temporally high-resolution MR exposures in a magnetic resonance system. The present application can in particular (but not exclusively) be used for generation of angiographic exposures by nuclear magnetic resonance.
2. Description of the Prior Art
In most MR applications it is desirable to obtain a high spatial resolution in the acquired MR images without extending the acquisition time too much. However, if the MR raw data set is acquired while adhering to the Nyquist condition (which means that the sampling rate is twice as high as the limit frequency to be depicted), the acquisition time automatically extends with increased spatial resolution since more raw data points must be collected in k-space.
In addition to the typical MR acquisition techniques, radially symmetrical k-space data acquisition techniques have also been used in order to accelerate the raw data acquisition overall.
A method in which radial k-space acquisition techniques are used for generation of angiography exposures is described in “Highly Constrained Backprojection for Time-Resolved MRI” by C. A. Mistretta in Magnetic Resonance in Medicine 55: pages 30-40 (2006). A number of under-sampled MR raw data is acquired with various radial projections. The different under-sampled raw data are added and a spatially high-resolution angiography image is generated by a filtered back-projection. An unfiltered back-projection is likewise implemented for each acquisition raw data set, wherein after normalization of the signal intensity the individual under-sampled MR images are multiplied with the averaged MR image in order to acquire a number of temporally high-resolution MR images overall. However, this method exhibits a number of disadvantages. For example, the calculation of the back-projection limits the application of this described method to radial acquisition techniques. The application of this method to other acquisition techniques (such as, for example, helical acquisition techniques or other acquisition techniques) is not possible. Furthermore, the method includes the step of calculating the back-projection from the averaged MR image that is in a Cartesian coordinate system. This method step is complex and very time-consuming.